The Biochemistry of Methane Monooxygenases

Selective methane (CH4) oxidation is extremely difficult chemistry. Methane monooxygenases (MMOs ) facilitate the biological conversion of CH4 into methanol in methanotrophs under ambient temperatures and pressures. Methanotrophs typically express the membrane-bound particulate form of MMO (pMMO ), but the soluble form of MMO (sMMO ) is often expressed under copper-limiting conditions. Numerous biochemical and biophysical approaches have been explored to elucidate the mechanisms of pMMO and sMMO over the past four decades, especially to examine the structures and the functional roles of the active sites. In this chapter, the biochemistry, including structural and functional features of MMOs, will be described. We first summarize the biochemical/biophysical studies that have led to the discovery of a unique tricopper cluster as the catalytic site in pMMO and culminated in the successful development of a biomimetic catalyst capable of mediating efficient CH4 oxidation at room temperature. We then review the spectroscopic, kinetic, and structural studies that have contributed to clarification of the catalytic mechanisms near the non-heme diiron active sites in the hydroxylase of sMMO, as well as the roles played by the regulatory protein and the reductase in this chemistry.